This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 198 11 101.0, filed on Mar. 13, 1998, the entire disclosure of which is incorporated herein by reference.
The invention relates to balancing of flexible rotors, more specifically, to ascertaining unbalance compensating values that represent an unbalance of an elastic rotor and are required for an unbalance compensation of the elastic rotor. The invention further relates to ascertaining coefficients which have an influence on the balancing of unbalanced elastic rotors, which normally have a rigid behavior in a low r.p.m. range.
In order to balance rigid rotors having a simple cylindrical shape and rotating at a low r.p.m., it is customary to measure the resulting unbalance of all individual unbalances generally in the left bearing or support plane and in the right bearing or support plane. If a balancing is necessary, the measured resultant of the unbalances are compensated in two balancing planes. The balancing planes define locations around the rotor where balancing weights are to be applied. As a result of the compensation by the attachment of balancing weights, a rigid rotor rotates free of vibrations otherwise caused by an unbalance and free of bearing forces. Generally, non-symmetric masses are distributed over the entire length of a rotor. As a result, internal bending moments remain in the rotor due to centrifugal forces produced by individual unbalances. In connection with elastic rotors, such internal bending moments can cause forces that rise with the square of the r.p.m., thereby leading to unpermissibly large deformations which in turn cause unbalance effects. This fact can cause a dangerous situation, especially when the operational r.p.m. approaches a bending critical r.p.m. which could without damping cause an infinitely large bending or a permanent deformation of the rotor.
Theoretically, a rotor or a shaft has infinitely many critical r.p.m.s. In order to evaluate or judge the vibration characteristic at a definite r.p.m. only those critical r.p.m.s are taken into account which cause corresponding bending configurations or xe2x80x9cmodal shapesxe2x80x9d that become troublesome. With regard to practical considerations, frequently it is sufficient for many rotor types to take but one critical r.p.m. into account which excites a rotor to assume a wave elasticity. However, under certain circumstances it may be necessary to take several critical r.p.m.s into account. A simple roller shaped or cylindrical rotor will bend in the shape of a U when approaching the first critical r.p.m. Such a simply rotor will bend into an S-configuration when approaching the second critical r.p.m. Such a simple rotor will bend into a W-configuration when approaching the third critical r.p.m. These bending configurations at critical r.p.m.s are referred to as xe2x80x9cmodal shapesxe2x80x9d of the rotor.
One must count on the occurrence of elastical deformations the more so the higher the operational r.p.m. is. Thus, it is the aim of a balancing operation to reduce unbalance forces to a tolerable level over the entire permissible operational r.p.m. range. Such unbalance forces involve rigid body forces and forces caused by the wave elastic deformation or deflection of the rotor. These forces must be reduced to a tolerable level by the balancing operation or unbalance compensation. In practice, there are known several balancing methods which take into account such wave elastic characteristics of rotors.
One such method has become known from an article by K. Federn entitled xe2x80x9cOverview Over Current Approaches, Guidelines, Standards, and Customary Methods for Balancing Wave Elastic Rotorsxe2x80x9d, VDI-Reports No. 161, 1971, pages 5 to 12. The just mentioned article describes a balancing in a plurality of measuring or sensing planes with a compensation in n+2 balancing or compensation planes. The balancing itself may be performed manually by applying the required balancing weights. In this connection it is necessary to perform a balancing operation in at least n+2 compensating or balancing planes if n-critical r.p.m.s are taken into account. According to Federn, first a conventional rigid body balancing operation is performed. Only then modal unbalances are eliminated with the aid of generally several test load sequences or test load runs. Such methods rely heavily on the experience and dexterity of the operator which effect the number of balancing sequences or runs required in order to achieve an optimal running characteristic at the operational r.p.m. of the rotor after the unbalance compensation is completed. As a rule, however, there are always a larger number of measuring sequence or runs required in order to achieve a good balancing result.
A textbook by W. Kellenberger, xe2x80x9cElastic Balancingxe2x80x9d, Berlin, 1987, pages 317 to 325, describes a computer aided balancing method with test loads or test weights applied to the rotor sample for ascertaining influence coefficients. The Kellenberger method eliminates or at least reduces the rigid body compensation and the wave elastic bending by compensation masses calculated in common for both types of deformations. For performing the method, an initial unbalance measuring sequence or test run is performed, whereupon at least as many further unbalance measuring sequences or test runs with test weights are required as balancing planes are provided. Thus, at least four unbalance measuring sequences are required when considering the first critical r.p.m. of the rotor to be balanced. According to the Kellenberger method the influence coefficients that are measured in the several measuring sequences with testing weights, are stored in the memory of the computer. Hence, these influence coefficients can be used for subsequent testing of the same type of rotors under favorable circumstances so that only one unbalance measuring sequence needs to be performed. In any event, for all first time balancing operations of rotors, these rotors must be loaded with testing weights and the number of measuring runs with testing weights must correspond to the number of compensation or balancing planes that are to be taken into account.
A report by R. Gasch and J. Drechsler, entitled xe2x80x9cModal Balancing of Elastic Rotors Without Applying Testing Weightsxe2x80x9d, VDI-Reports No. 320, 1978, pages 45 to 53, describes a method that ascertains the required compensating or balancing masses without the above described testing weight measuring sequences in order to compensate for wave elastic deflections of the rotor. Gasch et al. suggest to first perform a rigid body balancing followed by an unbalance measuring sequence all the way into the critical r.p.m. ranges that must be taken into account and to thereby measure the rotor deflections with the aid of displacement pick-ups positioned at predetermined rotor locations. With the aid of the stored or registered elastic deflections of the rotor shaft and with the knowledge of the modal shape and the corresponding generalized masses, it is possible to identify, computer aided, modal unbalance components and to calculate respective compensating weights. Such a method has the disadvantage that first a conventional rigid body compensation must be performed and only thereafter it is possible to eliminate modal unbalances by additional measuring and compensating processes.
German Patent Publication DE 4,133,787 A1 discloses another balancing method for elastic rotors, wherein the compensation masses required for balancing are ascertained without the use of measuring sequences with testing weights, for compensating rigid body unbalances and wave elastic deflections of the rotor. The rotor to be balanced is first run for one unbalance measurement at an r.p.m. at which the rotor exhibits rigid body characteristics, whereby first at least one unbalance measured value is obtained. Then, at least one further unbalance measured value is obtained for each support or bearing plane and for each modal shape to be compensated. The further measurement is made at an r.p.m. in the range of the inherent modal shape r.p.m. that must be taken into account. The so ascertained unbalance measured values are then processed in an evaluating computer which also takes into account rotor specific and/or bearing specific data for obtaining the mass or masses required for compensating the rigid body unbalance and the modal shape proportion to be taken into account in the balancing operation. The evaluating computer calculates a so-called force fingerprint for each bearing plane, whereby this force fingerprint is a constant value independent of the particular r.p.m. This force fingerprint provides information on the unbalance effect of the elastic rotor characteristic. The method of the just mentioned German Patent Publication 4,133,787 A1 requires providing, for example, through a keyboard, to the evaluating computer information regarding rotor specific data, bearing specific data such as dimensions, configurations, and the type of material of which the rotor to be balanced is made.
In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:
to provide a method for ascertaining information for the compensation of an unbalance in elastical rotors without the use of testing weights and substantially without the use of the rotor specific and bearing specific data mentioned above;
to avoid the separate ascertaining of rotor specific and bearing specific data;
to provide a method that permits a rapid balancing of elastic rotors based on values measured on the rotor and on calculated informations necessary for the balancing; and
to provide a testing method that can measure the effects of force applications when the rotor is in a standstill or when the rotor is rotating.
The inventor has recognized that it is possible to completely determine all influence coefficients that are required for a balancing operation also referred to herein as unbalance compensation if the rotor in question is excited by a force application in several compensation or balancing planes. By taking into account influence coefficients related to force excitation which are then transformed into unbalance influence coefficients by calculation and by a single unbalance run made to measure unbalance effects one can ascertain the unbalance compensating values without the need for taking into account specific system characteristics such as rotor specific data and bearing specific data. By using these calculated unbalance influence coefficients a balancing operation or unbalance compensation can be performed in a most simple manner by determining the required unbalance compensating weights on the basis of said coefficients and a single unbalance run.
According to the invention, there is provided a method for obtaining unbalance compensating values from a rotor having a rotation axis and an r.p.m. dependent elastic or rigid behavior, for balancing said rotor, said method comprising the following steps:
a) mounting said rotor in at least two bearing planes for rotating said rotor about said rotation axis,
b) determining a number of balancing planes required by said rotor and a number of modal shapes at which said rotor is to be balanced,
c) applying an excitation force to said rotor transversely to said rotation axis in each of said balancing planes for inducing in said rotor a vibration pattern,
d) sensing an excitation force spectrum or spectra caused by said excitation force that induced said vibration pattern,
e) picking-up a rotor specific response spectrum or spectra,
f) calculating ratios of said response spectrum or spectra to said excitation force spectrum or spectra for forming force influence coefficients (EFKF) resulting from said excitation force,
g) forming unbalance influence coefficients (EFKU) from said force influence coefficients (EFKF),
h) performing a measuring sequence for measuring rotor r.p.m. signals and rotor specific signals,
i) correlating said rotor specific signals to respective rotor r.p.m. signals for producing r.p.m. correlated rotor specific signals, and
j) calculating said unbalance compensating values from said r.p.m. correlated rotor specific signals and from said unbalance influence coefficients (EFKU). The above mentioned excitation force may be applied manually by a hammer blow or blows in one or more excitation planes, preferably when the rotor is at a standstill.
According to the invention there is further provided a method for obtaining unbalance influence coefficients for balancing a rotor having a rotation axis and an r.p.m. dependent elastic or rigid behavior, said method comprising the following steps:
a) mounting said rotor in at least two bearing planes for rotating said rotor about said rotation axis,
b) determining a number of balancing planes required by said rotor and a number of modal shapes at which said rotor is to be balanced,
c) applying an excitation force to said rotor transversely to said rotation axis in each of said balancing planes for inducing in said rotor a vibration pattern,
d) sensing an excitation force spectrum or spectra caused by said excitation force that induced said vibration pattern,
e) picking-up a rotor specific response spectrum or spectra, and
f) calculating ratios of said response spectrum or response spectra to said excitation force spectrum or spectra to form said unbalance influence coefficients by transformation. Here, the excitation force is also applied manually by a hammer blow or blows, preferably when the rotor is not rotating.
The invention makes it possible for the first time to ascertain the influence coefficients for obtaining or calculating balancing weights by simple measurements without first ascertaining data for the entire balancing system such as the above rotor and bearing specific data. Computer processing of these system data is also avoided. The influence coefficients are simply measured, processed in a computer for transformation and then used for ascertaining the balancing weights required for a balancing operation or unbalance compensation. A special advantage of the invention is seen in that the force application for exciting the rotor can be made either while the rotor is at a standstill or while the rotor rotates. Another advantage is seen in that several test runs with different testing weights applied to the rotor are no longer necessary.